In order to ensure during transmission of digital data that the transmission channel, normally having a capacitive component, is not charged up to a d.c. voltage value, the digital data are usually transmitted in an AMI-code or in form of a bipolar (=pseudoternary) pulse train for which the logical characteristic state 0 is represented by the signal voltage +U or -U with alternating polarity, for every successive 1-bit. Thereby, for an alternating sequence of logical 0 and 1 states in the pulse train, there is a smaller signal change or pulse train rate than for directly successive logical 1 states.
Due to the frequency dependent transmission characteristic of data transmission channels, digital pulse signals with different pulse train rates are often distorted during transmission to such a degree that at the end of the transmission path they can no longer be recognized as such. In order to regenerate the digital data, the transmitted digital pulse signals must first of all be equalized before they can be fed to a pulse shaper. For the automatic equalization of the distorted digital signals, generally an adaptive equalizer which automatically adapts its equalization characteristic to the unknown and often variable transmission characteristic of the transmission channel is used. Such an adaptive equalizer essentially consists of the variable and adaptable equalization network and a circuit arrangement controlling the equalization network.
The above discussed type of circuit arrangement for adapting the equalization network of an adaptive equalizer for digital pulse signals is already West German Patent Publication No. 29 08 588. It comprises a predistortion filter, an operational-sign-coincidence-auto correlator, and a correction circuit to guarantee control stability. This known circuit, however, has the disadvantage that it is very complicated in design, due particularly to the use of a sign-coincidence-auto correlator, so that on the one hand it is susceptible to functional errors and on the other hand it is expensive to produce.